Red, a young, orphaned boy with a grotesque, red left arm, was sold to the circus, but has no memories of his life prior to his purchase. Refusing to work as a performer, Red takes on odd jobs in exchange for food and a place to live, something that he manages very well despite his inability to move his left arm.
Despite his efforts, however, Red is largely ignored by the circus troupe, with the exception of a clown, Cosimov, who often blames and beats Red for his misfortune. Due to the arrival of a new, popular clown, his abuse at the hands of Cosimov begins to worsen as Cosimov loses his place as the most popular performer.
One winter day, after a show, Cosimov begins to beat Red. The ringmaster enters at the commotion, but Cosimov insists that he was trying to teach Red a lesson for lazing around. The ringmaster believes him and punishes Red by taking away his dinner.
That evening, Red walks outside and scooped up the snow to soothe the bruises on his face. Another boy comes up to him and, realizing that Red is from the circus, asks him what his act is. Red shows the boy his arm and the boy runs away in fear.
Later, Red sees Cosimov enter the tent and crash around. After Cosimov leaves, Red enters to clean up the mess Cosimov made, knowing that if any of the props are missing he would be to blame. As he cleans up, however, a dog jumps out from its sleeping place and steals one of the balls. Red runs after it. He chases it until he collapses and begins to lose hope that he will be able to recover the ball. The dog drops the ball on Red's head, however, and Red realizes it wants to play. They play catch for a little while and the rage and hatred that Red feels begins to melt a little.
For the next few days, Red decides to ignore the dog, knowing that someday it will go away with its master and never return. Despite the lack of attention it receives, the dog continues to follow Red around as he worked.
Cosimov then pulls aside Red and demands that he mix glass into the dogs food, knowing that if he kills the dog, he will become the number one clown once more; the clown currently holding the position would not be able to perform, as the dog is his partner. Red refuses, however, even as Cosimov begins to viciously beat him. Just as Red begins to lose consciousness, he hears barking.
Red awakens and fears that the dog was killed while protecting him. He is relieved, however, to find the dog performing with its owner and assumes that he had merely hallucinated the barking. He watches the two perform and begins to feel isolated and lonely as he witnesses the love and trust between the clown and his dog, and the love the audience feel for the radiant clown, feeling as always that it is a world far beyond his reach. Living an isolated, bleak life, until that day the dog befriended him, he finally realized that he, too, wanted to experience love. After the show, the dog approaches Red and, in his emotional turmoil, he kicks the dog. Shocked by what he has impulsively done, Red runs away.
Red does not see the dog for some time, but convinces himself it is for the better. One day, he sees the dog sleeping next to a heater covered with a coat and looking quite old. He hesitantly reaches out to pet it, worrying for its health. It wakes up and is happy to see Red, licking his left hand which no-one has ever dared touch. Red feels as if he has been forgiven and continues petting it, verbally praying over and over for it to get well. The dog is then called to perform and runs off, looking sprightly as ever, and Red is relieved when he realizes it might have simply been tired.
The next day is Christmas Eve, and the circus's performers all went to town to hand out flyers and advertise their circus. After doing some chores, Red sees the dog's owner sitting by a tree all alone. He goes to investigate and, to his shock, finds the dog lying in a freshly-dug hole, covered in bruises, clearly having been beaten to death. He begins to talk with the owner, but is unable to understand why he does not want revenge for something that Cosimov was clearly responsible for, and was not willing to even cry for the dog. As Red begins to cry, the man realizes that he was a friend of Allen's, his dog. Red cried until he fell asleep, and the new clown carried him back to the circus.
After Red wakes up, the clown set him down beside the stove, gave him his coat and hat, then handed him some flyers to hand out. He takes Red into town with him. As the clown performs, he suddenly invites Red to join him and his showmanship amuses the crowd. When Red holds out a hat for coins, the audience throws in what change they have and Red is shocked to have his skills recognized.
As Red is collecting coins, a red-haired priest asks Red for his name. Not having one, Red attempts to ignore him, but the priests persists and asks him if his name is Allen. When Red denies, the priest warns him not to get close to a man named Mana.
After collecting money, Red returns to the clown and asks if he is called Mana. Giving an affirmative answer, Mana asks how Red knew his name. After explaining about the priest, Mana suddenly looks uncharacteristically serious and runs off, looking around frantically. After a long search, Mana explains that the man may have been his little brother. Not giving up, Mana continues to search until Red is forced to push Mana out of the way of a carriage. The two fell into a mud puddle, and he drags Mana to a park to clean up.
Mana attempts to make small talk, but Red is still clearly miffed about Mana's carelessness. Red notices the most striking feature of Mana: His golden eyes. Mana suddenly tells Red that he is only seventeen. He claims that he woke up one day and looked at the mirror to find himself a middle aged man. He tells Red that he is looking for his little brother, but cannot find him, and that them both are being chased by a man named the Millennium Earl, who creates Akuma from humans, and if caught he will be killed. He fears that because he has suddenly become a middle-aged man, his brother will not recognize him, and so he began his life of wandering. A pessimistic Red suggests that perhaps Mana was abandoned by his brother. Mana does not respond, instead admiring the beautiful sunset and proclaiming how he dearly loves this beautiful world, though his smile looks melancholic.
Upon returning to the circus, Red sees Cosimov and is filled with rage. When he accuses Cosimov of killing Allen, Cosimov ignores him and accuses him instead. As the other circus members begin to glare at him, Red is overwhelmed with hatred and hits Cosimov with a plank, thinking of nothing but to avenge Allen's death. Cosimov exaggerates his injury, drawing the attention of the Ringmaster. The Ringmaster beats Red until the plank breaks and orders two of his men to throw Red into an animal cage. Knowing he is about to be killed, Red rushed at Cosimov again, determined to kill him in revenge for Allen. He attempts to hit Cosimov with his left arm, but hits Mana, who has stepped in the way, instead.
As Mana protects Cosimov, telling Red that he must not do something as "tragic" as killing, Red turns his anger to Mana and is infuriated by his ever-present smile. He begins to beat Mana over the head with his left arm, ignoring the light particles flying out of his head. Finally exhausted, Red stops and Mana tells him how lonely Allen was and how he wasn't able to do tricks at first. As Red realized how Allen was so similar to himself and that may be the reason Allen tried to befriend him, Mana thanks Red for being Allen's friend and caring so much for him, but begs him not to grief any further.
Red begins to worry about Mana's strange, broken speech patterns, and suddenly realizes that it must be because he was hit by Red's "cursed" arm. Red is then dragged away by the Ringmaster to be thrown to the animals, screaming for Mana. After he and the Ringmaster is gone, the Millennium Earl appears with a group of akuma and begins to kill everyone in the circus, while Mana mutter that grief will call for the Millennium Earl (which is why he refrains from grieving).The red-haired priest then arrives to kill the akuma with his gun, Judgment.
Meanwhile, Red is thrown into a cage with a carnivorous animal. Red is certain he will die until a yellow, round object flies in, knocking down the animal. It chews through the cage bars, allowing Red to crawl out. As he does, he saw the red-haired priest, who has knocked out the Ringmaster and the two men who had been dragging Red here. The priest, extremely furious, holds his gun to Red's head and prepares to kill him until the flying ball intervenes. The priest lets Red go, but tells him that all this happened because of him and orders him to disappear. The priest leaves and Red runs away from the circus, knowing he will be killed lest he stays, though it breaks his heart to leave Mana behind.
After wandering the town for a whole day, Red somehow finds himself walking back to the circus. He sees the crowd gathered there and heard news that everyone from the circus has disappeared save the Ringmaster and some survivors. The missing people left behind clothes which have holes like bullets in them, but there are no corpses. Red then discovered Mana, who is still covered in blood. Mana cheerfully greets him, calling him "Allen", apparently confusing Red with the dog. Mana suddenly begins to wonder why he is traveling. He said he feels like he is searching for something important but cannot remember what, and Red attempts to remind him of his missing brother. Red begins to wonder if Mana forgot because he hit him too much (As Red's arm is actually Innocence and Mana is later revealed to be a Noah like his brother Neah, it might be possible that the uninvoked Innocence damaged Mana's Noah memories).
Seeing Mana so forlorn at the loss of his memory, Red is consumed by guilt and despair. Realizing the old Mana will never return, Red begs Mana to take him along, offering to remember Mana's past for him. Mana promises the two of them will always be together, and the two set off together. Red finally realized that his reason for living is to remember the things Mana forgot because of him. As the night he played in the snow with Allen, and the night he performed with Mana was the first time in his life that he ever felt happiness, Red resolved that this time, he will make Mana happy by becoming Mana's "Allen".
The story concludes that that day was when Red becomes Allen, and Red's story ends here as Allen's story begins. Allen's next meeting with the priest and his golden golem, his training and his becoming an Exorcist was another story to be told in another time.